The purpose of the proposed research is to establish that cytoarchitectonically discrete, sexually dimorphic cell groups of the medial preoptic area (MPOA) are regulated by gonadal steroids in adulthood to affect reproductive function. Recent research indicates that the MPOA of the rat is composed of numerous distinct components. some of which are sexually dimorphic and which respond to gonadal steroids in adulthood with changes in morphology. Based on the assumption that a continued, detailed study of components of this relatively large brain region is essential in order to elucidate its functions, the specific aims of the proposed research are to establish that: 1) Steroid regulation of the neuropeptide galanin (GAL) takes place within sexually dimorphic cell groups of the MPOA, and 2) The activity of GAL and cholecystokinin octapeptide (CCK-8) within the medial preoptic nucleus (MPN) contributes to the regulation of reproductive behaviors and sex differences in their expression. Specific Aim I: Based on preliminary evidence, immunohistochemical methods can be used to identify GAL-immunoreactive(-I) cells within the MPOA, and this technique can be used to establish that GAL-I cells are localized within sexually dimorphic subcomponents, that gonadal steroids maintain levels of GAL activity within these cell groups, that gonadal steroid-concentrating cells can also be GAL-immunoreactive, and that there are sex differences in the distribution of GAL-I cells. Specific Aim II: Preliminary evidence indicates that unilateral microinjection of GAL into the MPN can decrease intromission latency (IL) in testosterone-treated gonadectomized males, and that MPN microinjection of sulphated CCK-8 (sCCK-8) has a dramatic facilitatory effect on lordosis behavior in estrogen-treated gonadectomized females and males. These findings are the basis for a series of experiments that will elucidate the role played by GAL in affecting male copulatory behavior and lordosis behavior in males and females, and the role played by testosterone and its metabolites in regulating the GAL- and sCCK-8-induced changes in these behaviors. In addition to the strong likelihood that this research will define a functional system within a discrete portion of the MPOA, the results may add significantly to our understanding of neural mechanisms responsible for sex differences in the expression of reproductive behaviors in adulthood.